1. Field of the Invention
The present invention relates generally to combustion chambers. Particularly, this invention relates to wood pellet and other fuel combustion chambers that provide for complete and efficient combustion. More particularly, this invention relates to a combustion cage for burning wood pellet or other solid fuels, that can be inserted into a fireplace, wood stove, or other similar device. Even more particularly, this invention relates to a combustion cage for burning wood pellet or other solid fuels to burn these fuels more completely and efficiently, a device that does not require any additional mechanical or electrical devices to enhance or promote combustion.
2. Description of the Prior Art
Wood pellet fuel is an economical, organic and renewable fuel that is convenient, easy and safe to use. Wood pellet fuel produces a cleaner burn than traditional cordwood or other fuel such as coal and costs about the same to use for an equivalent overall heat output. When compared to the cost of electric heat in the Northeastern United States, wood pellet fuel usually costs about two-thirds less for a comparable amount of heat. With wood pellet fuel there is no chopping, hauling, splitting or stacking of logs, which all require a larger amount of storage space than that required for wood pellet fuel. Also, there is no bark dust or coal dust that seems to permeate and cover every exposed surface in the home.
Wood pellet fuel is environmentally friendly because it is typically made from sawdust, a manufacturing by-product which is normally dumped into landfills. The process begins by pulverizing clean waste sawdust to a uniform size and then drying it to a specific moisture content, usually between 5-8% by weight. Completely seasoned cordwood, on the other hand can have a moisture level of 20% or more.
The pulverized dust is then forced through a press under high pressure to produce a typical wood pellet. When these pellets are used as fuel, they produce a clean, reliable heat with very low particulate emissions and with extremely low ash content. Generally, they produce ten to twenty times less ash than the most efficient wood stoves currently marketed.
Wood pellet fuel is normally sold by the ton and is usually put into easy-to-handle 20-50 lb. bags. Wood pellet fuel is usually available for home delivery or can be purchased at any number of nurseries, feed and supply stores, hearth shops, wholesale food operators, and mass merchants. A ton of wood pellets will deliver approximately 16-18 million BTU's of heat, while a conventional wood stove delivers approximately 6-10 million BTU's per cord.
Storage of wood pellet fuel is also more convenient than storing cordwood. A cord of wood occupies 128 cubic feet, while a ton of pellets is about 48 cubic feet. Wood pellet prices range from $100/ton in towns where the wood pellets are produced to $200/ton elsewhere. Average retail cost in the West is $140/ton, slightly more in the East.
Many alternatives to wood pellet fuel have become commercially available for industrial use. These alternatives are commonly referred to as biomass fuels and include shells and husks from hazelnuts, walnuts, almonds, and pecans. Additionally, cotton by-products, grass stubble, US currency, other paper products, and even garbage have been used as biomass fuels. Home use of these alternatives is not far off. It is a combination of convenience, environmental concerns, preservation of Hardwood forests, and cleaner burn that makes burning wood pellet and similar biomass fuels so attractive.
It was found that conventional wood stoves could not make use of the advantages of wood pellet fuel. The reason is that the wood pellet fuel smoldered after igniting instead of burning efficiently. Consequently, wood pellet stoves were designed to take advantage of these new solid fuels. A traditional wood pellet stove includes a hopper, an auger, a firebox or grate, a combustion fan and a heat exchanger which, respectively, store, feed, burn the fuel and transfer the heat into the room. The auger operates in a timed manner for controlling the delivery of the pellet fuel from the hopper into the firebox. This delivery is timed such that the wood pellets are fed into the firebox at substantially the same rate at which they are burned. This controlled delivery is required so that the fuel burns efficiently. Too much wood pellet fuel in the firebox causes the wood pellets to smolder and burn inefficiently. This is due to the reduced air flow that occurs through the wood pellet mass as the size of the wood pellet mass increases. The air flow through the wood pellet mass is simply a function of the accumulation of the wood pellet mass which is directly related to the size and shape of the wood pellets, causing the restriction of air flow. This occurs even with supplying a driven air flow to the combustion chamber. Efficient burning in pellet stoves as well as wood stoves is important for keeping creosote buildup in the chimney to a minimum. Excessive creosote in the chimney increases the likelihood of chimney fires which can lead to disastrous consequences. Thus, it was discovered early on, that in order to burn pellet fuel efficiently, the wood pellets had to be delivered in a controlled fashion.
A combustion fan provides a measured amount of air, the driven air flow previously mentioned. This can be accomplished in one of two ways by either mixing with the fuel or to blow the waste gases out of the exhaust causing air to be sucked through the combustion chamber, thus ensuring proper combustion. A heat exchanger transfers this heat into the room.
There are now pellet stoves that do not need a combustion fan to work properly. However, these pellet stoves still require controlled delivery of the pellet fuel to the firebox so that the wood pellets are substantially burned at the same rate as they are delivered to prevent the smoldering mentioned above. In addition, the size, number, and placement of openings in the bottom and side walls of the firebox must be "tuned" to insure that a clean burn operation occurs. Furthermore, precise matching of chimney flue height and diameter with the augured feed rate is also needed for the pellet stove to operate efficiently.
Wood pellet stoves and similar devices are expensive to buy, generally in the range of $1,500 to $3,000 each. It is this initial expense that prevents many homeowners from substituting their wood stoves for wood-pellet stoves and enjoying the benefits of wood pellet fuel.
Currently, some of the prior-art devices use fireboxes or baskets to burn coal or cordwood. Other prior art devices teach wood stove attachments for burning charcoal. Firebox devices or grates for burning wood pellets have also been designed. However, these devices have been specifically designed for use in pellet stoves, not wood stoves.
To date, firebox devices for burning wood pellets or other solid biomass materials have been unavailable for use in wood stoves because it was not possible before now to effectively and efficiently burn wood pellets or similar biomass fuels in a device other than a specially-designed wood-pellet stove.
U.S. Pat. No. 5,295,474 (1994, Whitfield et al.) teaches a combustion grate with rods for a pellet-fueled stove. The grate assembly supports the pellets for combustion and directs combustion gas into the fire. It includes a passive grate of unequally spaced rods designed to prevent the ash and clinkers from accumulating on the grate in amounts that could reduce the flow of combustion air into the fire. This particular device was designed for use in pellet stoves that use combustion fans for forcing the air flow up through the bottom of the grate assembly and which use a controlled fuel delivery system.
U.S. Pat. No. 5,133,266 (1992, Cullen) teaches a pellet burning heating device designed to efficiently burn pellet fuel without the use of a combustion fan system for introducing combustion air into, or extracting exhaust gases from, the pellet stove. The pellet stove uses a specially-designed firebox for receiving the pellet fuel in a timed manner. The level of the pellet fuel in the firebox at any given time is critical to the proper functioning of the device. It requires that the chimney flue height and diameter be particularly matched to the timed delivery of the augured fuel for efficient burning to occur. This firebox is specially designed with "tuned" apertures for operation in a specially designed pellet stove.
U.S. Pat. No. 3,266,478 (1964, Booth) teaches a barbecue apparatus which is used for burning charcoal. This barbecue apparatus consists of a cabinet having an upper chamber and a lower chamber, a grill, an air blower, and a plurality of rectangular combustion baskets. The upper chamber is further divided into a plurality of compartments for accepting one of the rectangular combustion baskets. Each combustion basket has a grate bottom, perforated sides above a peripheral projecting flange which is in spaced relation above the grate bottom, handles, and an open top. The projecting flange of each basket seals off the upper chamber from the lower chamber so that air flow for combustion is permitted only through the grated bottom. This device is expensive to make and relies on a forced air flow to achieve an even burning of the charcoal held within each basket.
U.S. Pat. No. 4,289 (1845, Webb) teaches a portable stove that includes a casing, a grate in the shape of an inverted pyramidal form which is supported by brackets cast on the inside of the casing for providing heat on all sides of the portable stove, and hinged side doors that are supported when in a horizontal position and that allow direct access to the sides of the grate for cooking.
U.S. Pat. No. 790,166 (1905, Wood-Allen) teaches a charcoal-burning attachment for stoves consisting of a perforated sheet metal plate in the shape of a semi-circular trough. The semi-circular trough is designed to allow the fuel to fall towards the center of the trough as it burns, thus assuring that the fuel is all burned up. The design of the charcoal-burning attachment is specially made to fit any size of firebox of an ordinary cooking stove or range, not a heating wood stove. This device also was not designed for using wood pellet and other biomass solid fuels.
U.S. Pat. No. 984,200 (1911, Eastes) teaches a base burning heating stove for burning soft or hard coal and wood. This heating stove utilizes a cylindrically-tapered firebox which has perforated sides and two heavy iron crossbars at the bottom. Pivotally attached to the crossbars below the perforated sides is a shaking grate. The perforations in the sides are large openings through which a poker may be inserted for stirring the fire. The fire pot is specifically designed for use with the base-burning heating stove.
Therefore, what is needed is a combustion cage that is inexpensive to produce and easy to manufacture. What is also needed is a combustion cage that is easy to clean and repair. What is further needed is a combustion cage that can be made into any functional size for use in wood stoves, fireplaces and other combustion environments. What is still further needed is a combustion cage that is capable of providing complete, efficient and effective combustion of wood pellet or other solid biomass fuels without the aid of other mechanical devices such as combustion fans and timed fuel-delivering augers. Finally, what is needed is a combustion cage that is easy to use and that produces a consistent burn.